Polyurethane reaction mechanism

In order to demonstrate the use of laser flash photolysis in elucidation of the MDI based polyurethane photolysis mechanism, three polyurethanes, two aryl biscarbamate models, an aryl monocarbamate model, and an aromatic amine were selected. Two of the polyurethanes are based on MDI while the third is based on TDI (mixture of 2,4 and 2,6 isomers in 80/20 ratio). The MDI based polyurethanes all have the same basic carbamate repeat unit. The MDI elastomer (MDI-PUE) is soluble in tetrahydrofuran (THF). The simple polyurethane (MDI-PU) based on MDI and 1,4-butanediol is used in the tert-butoxy abstraction reactions since it does not contain a polyether backbone. (See page 47 for structures of polymers and models.)... 

FIGURE 7.3 Common amine reaction mechanisms for epoxy and polyurethane resins. 

The tensile properties of polyester-based thermoplastic polyurethanes were studied as a function of the time of exposure of the plastics to water, methanol, methanol-water, methanol-Isooctane, and methanol-water-lsooctane. The resulting decrease In the tensile properties of the plastics was attributed to reaction of the plastics with water and methanol. As Indicated by the decrease In properties, reaction with methanol Is Initially faster, but the reaction rate with water Increases with time — presumably because of the autocatalytic nature of the reaction. Nuclear magnetic resonance spectroscopy Indicated that the reaction mechanisms with methanol and water were transesterification and hydrolysis, respectively. 

Most studies of the kinetics of the isocyanate-hydroxyl reacfion have been done in systems composed of monofunctional reactants in various solvents (2 ). Even in these ideal systems, which have little resemblance to the more complicated polyurethane formulations, the reaction mechanism and kinetics are not well understood especially for the catalyzed reaction. This coupled with the added complexities encountered in polyurethane systems requires empirical determination of kinetic data if conversion during polymerization is to be predicted. A few kinetic studies on simple polyurethane systems have been reported (3, 4 ). Infrared spectroscopy was used to measure reaction rates in low catalyst formulations (3) while adiabatic temperature rise methods have been used to study fast systems (3 4, 5 ). 

Zabalov, M. V., Tiger, R. P., and Berlin, A. A. Reaction of Cyclocarbonates with Amines as an Alternative Route to Polyurethanes A Quantum-Chemical Study of Reaction Mechanism, Doklady Chemistry 441, no. 2 (2011) 355-360. 

Problem 5.7 Polyurethanes having the characteristic linkage -NHCOO- are formed when a diol undergoes condensation polymerization with a diisocyanate (see Table 1.2). The polymerization kinetics is found to be of third order at very low concentrations but of second order at higher concentrations. This led to the proposition (Baker et al., 1949) of the following reaction mechanism ... 

Two types of addition polymerization exist that differ in their- reaction mechanism and their kinetic behavior from each other and from polycondensations. The first proceeds as a step reaction, whereas the second one shows all characteristics of a chain reaction. The step-reaction type of addition polymerization may be exemplified by the polymerization of ethylene oxide in the presence of traces of water (see Fig. 15-26). The chains grow proportionally to the reaction time, and each intermediate product is a stable, saturated molecule. The main difference between this reaction and a polycondensation is the absence of any reaction proddct that is split off during the process. On the other hand, it differs distinctly from the second type of addition polymerization in which the polymer chain is built up instantly after an initiator has been formed and where the intermediates are unstable species. Some addition holymers of the step-reaction type have become industrially important. Foremost among them are poly-siloxanes, polyethylene oxides, and polyurethanes. 

Although these definitions were perfectly adequate at the time, it soon became obvious that notable exceptions existed and that a fundamentally sounder classification should be based on a description of the chain-growth mechanism. It is preferable to replace the term condensation with step-growth or step-reaction. Reclassification as step-growth polymerization now logically includes polymers such as polyurethanes, which grow by a step-reaction mechanism without elimination of a small molecule. 

Reaction Mechanism of Hydroxymethylated Resorcinol Adhesion Promoter in Polyurethane Adhesives for Wood Bonding... 

For thermosets with conqilicated reaction mechanisms or where the conqiosition and functionalities of the molecules in the resin mixture are unknown, an enqiiricd ai oach is necessary. In order to describe the viscosity of polyester matrices Kenny et al. (3) adopted a model similar to file one originally used by Castro and Macosko for polyurethanes viscosity (50) ... 

Kinetics and postcure reactions mechanism in polyurethane elastomers achieved with excess of NCO groups... 

The properties of PBT based copolymers depend on the composition of the reaction mixture. The composition is influenced by the reaction mechanism and kinetics, which are further influenced by the choice, purity, and the concentration of the main reaction components, by the presence and proper choice of the amount of catalyst and by the reaction temperature. The mechanism and kinetics of polyesterification, polyamidation, polyurethanes and many more have been investigated in the past as well as today . 

Kinetic studies have been made on the thermal decomposition of a poly(oxypropylene)triol-toluene di-isocyanate copolymer foam. Following a diffusion rate-controlled step, the cellular structure collapses to a viscous liquid and degradation then occurs on a random scission basis. Products of degradation of A-monosubstituted and A A-disubstituted polyurethanes have been analysed by direct pyrolysis in the ion source of a mass spectrometer. The mono-substituted polymers depolymerize quantitatively to di-isocyanates and diols, whereas the disubstituted materials decompose selectively to secondary amines, olefins, and carbon dioxide. The behaviour of the monosubstituted polymers has been confirmed in an i.r. study of the degradation of model compounds. A study of the thermal degradation in vacuum of polyurethanes prepared from butanediol, methylene bis(4-phenylisocyanate), and hexanedioic acid-ethylene glycol-propylene glycol polyesters has been reported and reaction mechanisms proposed. ... 

M. Zhaoguo, D. Zhiwen, and P. Xiaoyan, Mechanism of polyurethane modified collagen and its application Preparation of oil-based collagen and its reaction mechanism with... 

The fiber cladding is stripped where the fiber is inside the sample. A schematic diagram where a polymer reaction mechanism is investigated is shown in Figure 15.21. The reactants were poured into the sample container and the spectra were collected at the rate of a spectmm every 1 to 5 s. The data collected during the formation of a polyurethane foam reaction are shown in Figure 15.22. 

Beyond the slow reaction mechanisms of polyurethane chemistry, other reaction mechanisms take place in a fraction of a second. What makes these reaction mechanisms further attractive is that their trigger can be controlled. In particular, they can be induced by radiation energy, enabling a cross-linking process on command. 

The reaction mechanism occurring in the formation of polyurethanes was presented in Chapter 7, and the preparation of prepolymers was presented for linear PUR (Chapter 15). The structure of the prepolymers used to prepare three-dimensional PUR is simpler than that of linear ones indeed, with the former the cohesion of the material is mainly due to the presence of cross-links. 

Step-growth polymerization is characterized by the fact that chains always maintain their terminal reactivity and continue to react together to form longer chains as the reaction proceeds, ie, a -mer + -mer — (a + )-mer. Because there are reactions that foUow this mechanism but do not produce a molecule of condensation, eg, the formation of polyurethanes from diols and diisocyanates (eq. 6), the terms step-growth and polycondensation are not exactly synonymous (6,18,19). 

The polyurethane foam used for home insulation uses methanediphenyl-diisoevanate (MDI) as monomer. The MD1 is prepared by acid-cataly/ed reaction of aniline with formaldehyde, followed by treatment w ith phosgene COCl2- Propose mechanisms for both steps. 

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